1. Field of the Invention
The present invention relates to an image forming apparatus that detects the surface potential of a photosensitive drum as an image bearing member and controls operations thereof based on a detection result.
2. Description of the Related Art
As an image forming apparatus that forms an image on a recording material, the configuration and general operation of an electrophotographic printer will be described with reference to FIG. 14. The printer illustrated in FIG. 14 includes a photosensitive drum 101 as an image bearing member, a semiconductor laser 102 as a light source, a rotational polygon mirror (also referred to as a polygonal mirror) 103 that is rotated by a scanner motor 104, and a laser beam 105 that is irradiated from the semiconductor laser 102 and scans the surface of the photosensitive drum 101.
A charging roller 106 acts as a charging member for uniformly charging the photosensitive drum 101. A development unit 107 is for developing an electrostatic latent image formed on the photosensitive drum 101 with toner. A transfer roller 108 acts as a transfer member for transferring a toner image developed on the photosensitive drum 101 by the development unit 107 onto a recording material. A fixing roller 109 acts as a fixing member that heats the toner image transferred onto the recording material to fuse the toner image on the recording material.
A feeding roller 110 acts as a feeding member that rotates to feed a recording material from a cassette in which the recording material is stacked onto a conveyance path. The cassette has a function of identifying the size of the recording material. A manual feeding roller 111 feeds a recording material from a manual feed port, which is a separate feed port to the cassette. Conveyance rollers 114 and 115 convey the fed recording material.
A recording material detection sensor 116 is for detecting a leading edge and a trailing edge of the fed recording material. A pre-transfer conveyance roller 117 feeds the conveyed recording material to a transfer unit configured of the photosensitive drum 101 and the transfer roller 108. A synchronization sensor 118 is for synchronizing the writing of the electrostatic latent image (image) on the photosensitive drum 101 and the recording material to be conveyed with the fed paper. Further, the synchronization sensor 118 also measures the length in the conveyance direction of the fed recording material. A discharge detection sensor 119 is for detecting the presence of a fixed recording material. A discharge roller 120 is for discharging a fixed recording material out of the apparatus.
A flapper 121 switches the conveyance destination (discharge out of the apparatus, or convey to a two-sided unit) of the recording material on which an image has been formed. A conveyance roller 122 is for conveying a recording material conveyed to a two-sided unit to a reversing unit. A reversal detection sensor 123 detects the leading edge and the trailing edge of the paper conveyed to the reversing unit. A reversing roller 124 reverses the recording material and conveys the recording material to a re-feeding unit by sequentially switching between forward direction rotation and reverse direction rotation.
A re-feeding sensor 125 detects the presence of a recording material at the re-feeding unit. A re-feeding roller 126 re-feeds the recording material at the re-feeding unit into a conveyance path for conveyance toward the transfer unit.
Next, a block diagram illustrating the configuration of a control circuit for controlling operations of the above-described printer will be described with reference to FIG. 15. In FIG. 15, a printer controller 201 rasterizes image data sent from a (not illustrated) external device, such as a host computer, into the bit data necessary for printing by the printer, reads information in the printer, and controls operations based on that information.
A printer engine control unit 202 controls operation of each unit in the printer engine based on instructions from the printer controller 201, and sends information in the printer engine to the printer controller 201. A paper conveyance control unit 203 drives and stops the motors (conveyance roller etc.) for feeding and conveying the recording material based on instructions from the printer engine control unit 202.
A high-voltage control unit 204 controls the output of high voltages in the various steps such as charging, development, and transfer in the electrophotographic process based on instructions from the printer engine control unit 202. An optical system control unit 205 controls the driving and stopping of the scanner motor 104, or the turning on of a laser beam based on instructions from the engine control unit 202.
A fixing device temperature regulation control unit 207 is for regulating the temperature of the fixing device to a temperature specified by the printer engine control unit 202. A two-sided unit control unit 208 controls operation of a two-sided unit that can be attached/detached from the printer main body. The two-sided unit control unit 208 performs a paper reversal operation and a re-feeding operation based on instructions from the printer engine control unit 202, and simultaneously notifies the printer engine control unit 202 of those operation states.
Next, a schematic configuration of a typical charging voltage application circuit will be described with reference to FIG. 16. This charging voltage application circuit is a high-voltage circuit for applying a high voltage to the charging roller 106. In FIG. 16, a circuit 401 generates a direct current (DC) voltage (also referred to as DC bias) applied to the charging roller. A voltage setting circuit unit 402 is a circuit whose setting value is changed when a pulse-width modulation (PWM) signal is received. The charging voltage application circuit illustrated in FIG. 16 also includes a transformer drive circuit unit 403 and a high-voltage transformer 404.
A feedback circuit unit 405 detects the value of the voltage applied to the charging roller 106 using a resistor R71, and transmits the detected voltage value to the voltage setting circuit unit as an analog value. Then, based on this analog value, a constant voltage is applied to the charging member.
Based on such a configuration, by performing a series of controls, a constant voltage can be applied to the charging roller acting as a charging member. Japanese Patent Application Laid-Open No. 6-3932 discusses such a technology, in which a constant voltage is applied to a charging roller.
The voltage at which discharge starts for the photosensitive drum acting as an image bearing member by applying a high voltage to the charging roller is known to change based on, for example, the temperature and humidity of the environment in which the printer is set, and the film thickness of the photosensitive drum.
The fact that the characteristics of the discharge start voltage to the photosensitive drum are different based on the environment (temperature and humidity) and the film thickness will now be described with reference FIG. 17. In FIG. 17, the horizontal axis represents the voltage applied to the photosensitive drum, and the vertical axis represents the current flowing to the photosensitive drum. The point at which the current starts to flow is the voltage at which discharge started. It can be seen from FIG. 17 that since the discharge voltage varies, the potential (Vd) of the photosensitive drum surface is not constant even if a constant voltage is applied to the photosensitive drum.
Further, since the sensitivity of the photosensitive drum surface to the laser beam also varies based on the environment (temperature and humidity) and the film thickness of the photosensitive drum (thickness: large (thick)>medium (standard)>small (thin)), the surface potential of the photosensitive drum also varies after laser irradiation even if a constant laser light amount is irradiated on the photosensitive drum.
FIG. 18 illustrates the fact that the potential (VL) of the photosensitive drum after irradiation by the laser beam exhibits different characteristics based on differences in the film thickness of the photosensitive drum. In FIG. 18, the horizontal axis represents the light amount of the laser beam, and the vertical axis represents the potential of the photosensitive drum after irradiation with the laser beam (expressed as VL). Based on this data, it can be seen that that the potential (VL) of the photosensitive drum after irradiation with the laser beam is not constant even if a constant laser light amount is irradiated on the photosensitive drum.
Further, as a photosensitive drum characteristic, fluctuation (also referred to as drum memory) in the surface potential of the photosensitive drum irradiated with light, such as by irradiation with a laser beam, also occurs. Normally, although the surface potential of the photosensitive drum is ideally 0 V after charge on the photosensitive drum surface has been removed, since the potential is negative due to the influence of this potential fluctuation, variation in the surface potential of the photosensitive drum after irradiation with the laser beam occurs.
Conventionally, to correct this variation, for example, a storage element (a non-volatile memory) has been provided in the cartridge as a replaceable part in the photosensitive drum for storing information indicating the sensitivity of the photosensitive drum, and application voltage values based on the usage amount of the photosensitive drum. Based on the information in the storage device, the high voltages (charging voltage and development voltage) are variably controlled to match the sensitivity and the usage amount.
Further, the light amount of the laser beam has been also variably controlled. However, the increases in conveyance speed and drive speed during printing and the increases in the capacity of the cartridges containing the toner made to improve the productivity of the printer have made it more difficult to sufficiently correct this variation with conventional technology that performs control based on information about the storage element.
The reason why it is difficult to correct this variation will be described referring to FIG. 19. In FIG. 19, if the potential after a photosensitive drum has been charged by a charging roller is Vd, the potential after exposure by a laser beam is VL, and the development potential when developing with a development unit is Vdc, the potential difference Vdc−VL during a normal period and the potential difference Vdc−VL when the sensitivity of the photosensitive drum has deteriorated are different. Since it is difficult to correct this potential difference, density unevenness occurs in the image.